FIELD OF THE INVENTION
The invention relates to a process for producing wear-resistant boride layers on metallic material surfaces.
Wear-resistant boride layers are usually produced in practice using solid boriding agents, for example in the form of powders, pastes or granules.
A disadvantage of these processes is that they are labor-intensive in terms of packing, unpacking and cleaning the parts. Cleaning is carried out using a combination of washing and brushing or abrasive-blasting. Since the powders, pastes and granules can be used only once, problems also arise in disposing of the spent boriding agents.
In addition, the use of liquid boriding agents, for example in the form of salt melts, is also known. However, all these processes have not been able to become established owing to the problems generally associated with salt baths, viz. those relating to safety of handling, cleaning of the parts after treatment and disposal of the baths or their waste products.
In the past, there have been various attempts at boriding using gaseous boriding agents (CVD processes). When using organic boron compounds (trimethylboron, trialkylborons), carburization occurred predominantly instead of boriding; when diborane is used, safety problems occur because of the extreme toxicity and the risk of explosion.
The use of a boron trichloride as boron donor medium has not been able to become established because of process-inherent problems in layer formation. The cause of these problems is the hydrogen chloride formation which always occurs in boriding using BCl.sub.3 /H.sub.1 mixtures.
In the boriding of ferrous materials using boron trichloride, the following fundamental reactions occur: EQU 2BCl.sub.3 +3H.sub.2 +2Fe.fwdarw.2FeB+6HCl EQU 2BCl.sub.3 +3H.sub.2 +4Fe.fwdarw.2Fe.sub.2 B+6HCl
The hydrogen chloride gas formed in boriding using BCl.sub.3 reacts with the iron of the base material to form volatile iron chlorides: EQU 2HCl+Fe.fwdarw.FeCl.sub.2
The iron chlorides have high vapor pressures at the treatment temperatures in the range 500.degree. C.-1200.degree. C. which are employed, resulting in substantial, ongoing evaporation of iron chloride. This leads to hole formation between boride layer and substrate, as is always criticized in the case of the BCl.sub.3 process. Suppression of the hole formation is only possible if one succeeds in generating an impermeable boride layer within a very short time at the beginning of boriding. This is technically difficult in that to the present time it cannot be achieved reliably and reproducibly.
Apart from the purely thermal variant of CVD boriding, work on plasma-aided boriding (PACVD boriding) is also known. Hitherto, only diborane and boron trichloride have been used in this process variant, accompanied by the disadvantages which are already known from thermal CVD. An overview of the processes mentioned may be found in the review "Engineering the Surface with Boron Based Materials", Surface Engineering 1985, Vol. 1, No. 3, pp. 203-217.
It is therefore an object of the invention to provide a process for producing wear-resistant boride layers on metallic materials, which process does not suffer from the abovementioned disadvantages.